334
UNIT 3
Organ Systems
Thyroid Gland
Follicle
Follicle
Follicle
Colloid
Colloid
Colloid
Septum
Septum
Septum
Connective
Connective
tissue septum
tissue septum
Connective
tissue septum
A
Figure 17-8A.
Thyroid follicles, thyroid gland.
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3
70
The
thyroid gland
is derived from the developing endoderm of the
foramen cecum of the tongue; it has two lobes and is one of the
largest endocrine glands. Connective tissue
septa
divide the thyroid
gland into lobules. Each lobule consists of numerous
thyroid fol-
licles
. The thyroid follicles are the main functional components of
the gland; they synthesize and release
T
3
and
T
4
. Each follicle is F lled
with
colloid
, which is a gelatinous substance containing the stored
form of T
3
and T
4
. The follicular cells are usually simple cuboidal
cells but may change to simple squamous (inactive) or columnar
cells (active) depending on their states of secretion (±ig. 17-8B).
The thyroid hormones play an important role in regulating the
basal metabolic activity of the body.
Iodine
is required for for-
mation of thyroxine;
iodine de± ciency
can lead to the develop-
ment of
thyroid goiters
(
nodules
).
CLINICAL CORRELATION
Figure 17-8C.
Hashimoto Thyroiditis.
H&E,
3
55
Hashimoto thyroiditis
is a chronic autoimmune disease, char-
acterized by enlargement of the thyroid gland (
goiter
) and
gradual failure of thyroid function. Hashimoto thyroiditis is
the most common cause of hypothyroidism in the United States
and primarily affects women. Autoantibodies against thyroid
antigens, genetic susceptibility, and environmental factors are
believed to play a role in the development of the disease. The
signs and symptoms related to hypothyroidism include fatigue,
increased sensitivity to cold, pale skin, constipation, muscle
pain and weakness, and weight gain. Histologically, inF ltrating
lymphocytes form lymphoid follicles (lymphatic nodules) with
germinal centers within the thyroid parenchyma. Some thyroid
follicle cells show
Hurthle cell
change with abundant eosino-
philic cytoplasm. Thyroid hormone replacement therapy is the
treatment for the disease. Surgery may be indicated if enlarge-
ment of the thyroid gland causes compression of the airway.
Follicular
Follicular
(cuboidal)
(cuboidal)
cells
cells
Follicular
(cuboidal)
cells
Colloid
Colloid
Colloid
Parafollicular
Parafollicular
cells
cells
Parafollicular
cells
Follicular
Follicular
(squamous)
(squamous)
cells
cells
Follicular
(squamous)
cells
B
Figure 17-8B.
Parafollicular cells, thyroid gland.
H&E,
3
702
Another type of endocrine cell located between the follicules of
the thyroid gland is called a
parafollicular cell
. These cells are also
called
clear cells
or
C cells
and are commonly located within the
interstitial connective tissue septa. Parafollicular cells produce
calci-
tonin
, which inhibits osteoclasts from resorbing bone tissue, thereby
decreasing blood calcium levels. High blood calcium levels stimulate
parafollicular cells to secrete calcitonin. Parafollicular cells are rela-
tively large cells with round nuclei and pale cytoplasm. They can be
found scattered beneath the follicular cells or in small groups in the
interstitial connective tissue between the follicles, as shown here.
Graves disease
is an example of
hyperthyroidism
in which exces-
sive amounts of thyroid hormones are secreted by follicular cells
(see ±ig. 3-5C). In
hypothyroidism
, thyroid glands produce
abnormally low levels of thyroid hormones, such as in
Hashim-
oto thyroiditis
.
Thyroid
follicle
Hurthle cell
change
Lymphocytes
in germinal
center
C
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